This invention relates to a water disinfection method wherein composite bactericidal adsorption materials are used for the treatment of drinking water from the tap and other fresh water sources.
During the last two decades, various adsorbents have been used to disinfect water. The most common of them are activated carbon materials and ion exchange resins with bactericidal compounds, such as iodine, bromine and silver. The strong bactericidal properties of iodine and silver make them ideal disinfectants for small scale water supply systems.
It is known in the prior art of drinking water disinfection to use powdered activated carbon with particle sizes from 0.1 to 2000 xcexcm, as a carrier for silver in metallic form or as a nitrate salt (West German Patent Application No. 3229340, published 1984, B01I20/20), or to use activated carbon impregnated with silver salt (USSR Patent No. 971 464, published 1982, B01I20/20).
For the simultaneous disinfecting and purification of drinking water it is known to use a composition of coarse and fine carbon fibers, including carbon fibers activated with metal salts; for instance, silver in an amount of 0.01 to 8% deposited thereon as a bactericidal additive (Switzerland, Patent No. 556,680, published 1974, B01D39/00).
The disinfection of drinking water by passing it through the above described activated materials which were treated with silver salts was not sufficiently efficient due to the fact that the improvement in the bactericidal properties was effected by silver ions released into water in the course of the treatment. To ensure longevity of the bactericidal properties, it is necessary to treat said materials with concentrated silver salt solutions, which may adversely affect human health if imbibed.
U.S. Pat. No. 4,555,347 teaches the use of filtration material in the form of activated carbon and iodine crystals for water disinfection. However, the method, which is based on releasing iodine into water, can not be used for continuous consumption.
In order to eliminate bacteria in drinking water, it is known to use anion-exchange resin treated with silver nitrate solution (U.S. Pat. No. 2,434,190), and to use a cation exchange resin treated with a silver salt solution (U.S. Pat. No. 2,692,855).
In the case of long term use of bactericide containing ion-exchange resins, water also accumulates large amounts of silver ions. It happens in the same manner as with activated materials. In addition, in the course of disinfection, the resin becomes contaminated and loses its ion-exchange properties.
U.S. Pat. No. 3,817,860 discloses a water disinfection method where water is brought into contact with layers of an iodine containing resin and treatment by silver salts. U.S. Pat. No. 5,366,636 and the Journal of Water Chemistry and Technology, USSR, 1989, vol. 11, No. 2 disclose methods wherein the water passes through layers of iodine and silver containing ion-exchange resins. In the abovementioned cases, iodine is released into the water in the course of the drinking water disinfection, and then iodine is removed due to binding into the insoluble AgI compound.
According to the U.S. Pat. No. 5,366,636, water was passed through a porous, granular, iodine containing anion-exchange resin. As water actively contacted the resin, the iodine was released into the water. Then the treated water was passed through the porous granules of a chelating Ag-Chelex resin, which contains iminodiacetate groups and bound silver ions. The silver ions react with the iodide ions forming insoluble silver iodide. This method has a disadvantage, because to be effective large amounts of iodine must be released into the water from the anion-exchange resin. Then it is necessary to trap this iodine in the subsequent layers of adsorbents. In the likely event that there are channeling effects in the subsequent layers of adsorbents, or, if the adsorbent becomes saturated by other contaminants, iodine will leak into the purified water.
The process of water disinfection by passing it successively through layers of iodine containing anion exchange resin, synthetic activated carbon and macroporous strong acid cation exchange resin treated with silver nitrate solution allows to efficiently disinfect water of microorganisms (E. Coli). However, it implies the use of high concentrations of bactericidal components. As a result, molecular I2 and Ag+ ions remain in the water and have to be purged out in the course of subsequent treatment (Journal of Water Chemistry and Technology, USSR, 1989, vol. 11, No. 2).
There is also known in the prior art a method of water disinfection using filtering material composed of an ion-exchange resin mixture. The essence of the method is in passing water through the mixture of ion-exchange resins (99%) and bacteriostatic resin (1%). The bacteriostatic properties of the resin are related to the metallic silver grains present on the surface and inside the granules of the resin. The mixture of resins prevents biomass development in the ion-exchange filter and the infiltration of bacteria into the water (xe2x80x9cEau et Ind.xe2x80x9d, 1981, No. 58, 88 -90). However, despite the mentioned merits of the method, the silver ions are still washed out of resin in the course of time, getting into the water and, being accumulated in it, adversely affect human health.
Although much attention has been given to the issues of drinking water disinfection, and while iodine and silver have been used in the prior art, it was not known how to avoid, with the course of time, leakage of the iodine and/or the silver into the filtered water. For instance, if water contains an increased concentration of dissolved salts (high hardness), silver is quickly washed out due to the ion exchange mechanism. Problems arise during disinfection of drinking water with iodine, because it is necessary to enrich the water with a large quantity of iodine (at least 1 mg/l). The iodine has to remain in contact with the water for a long time, followed by the subsequent removal of iodine from the outflowing water. At iodine concentrations in water exceeding 4 mg/l, water acquires a distinct iodine odor. Long term consumption of iodinated water affects the thyroid gland. Secondly, when water passes through bactericide layers of resins, with time there is an accumulation of bacteria and biomass in the layer that does not contain bactericides, and bacteria subsequently infects the drinking water.
An object of this invention is the development of a new drinking water disinfection method which ensures disinfection reliability and efficiency with the preservation over time of the degree of water purification in removing heavy metal ions, organic matter and the improvement of taste and odor.
The present invention solves the problems of the prior art, and comprises filtration of drinking water through a composite material containing uniformly distributed granules of iodine containing anion exchange resin, granulated activated carbon, amphoteric fibers, and silver containing adsorbent; which will generally be a granular cation exchange resin such as C249 from Sybron, USA with Ag+ ions thereon. Silver containing adsorbent in granular form is preferred over the fibrous form, because it has been found that granules release silver more easily by ion-exchange mechanism.
An important aspect of our invention is to maintain the exterior surface area of the iodine containing anion-exchange resin granules at not more than 1% of the exterior surface area of the amphoteric fibers, and is preferably kept at less than 0.2% of the exterior surface thereof. In addition, in accordance with this invention, the total equivalent content of silver in the said composite material must exceed the equivalent content of iodine therein. While almost any excess of silver over iodine can be used (e.g., as little as 0.1%), desirably the excess equivalent content of silver will be more than about 10 percent. Preferably, the excess equivalent content of silver will be from about 15 to 30 percent greater than the equivalent content of iodine. It is within the scope of the invention to use as much as 50% excess or more. A higher excess of silver can be used where the water to be treated contains chlorides or other ions which are capable of reacting with the Ag+ ions to form substantially insoluble compounds. Upon contact with water containing halides, a layer of practically insoluble silver halogen compounds will be formed on the surface of the iodinated anion-exchange resin.
In a first embodiment of the invention, the composite material of the invention is comprised of:
In another embodiment of the invention, the composite material is comprised of:
The amphoteric fibers in the first and second embodiments of the composite material of the present invention comprise activated carbon fibers, ion-exchange polymer fibers and mixtures thereof. Useful activated carbon fibers will have an adsorption capacity of methylene blue of at least 360 mg/g, lengths of 0.2 to 10 mm, and diameters of 1 to 20 microns (preferably diameters of 5 to 10 micron). Suitable activated carbon fibers can be produced using procedures such as those described in U.S. Pat. No. 5,521,008.
A preferred ion-exchange polymer fiber is modified polyacrylonitrile, which can be manufactured using conventional techniques involving alkaline hydrolysis of polyacrylonitrile in the presence of cross-linking agents. Suitably, the ion-exchange polymer fibers will have an ion-exchange capacity of basic groups of at least 1 meq/g, and of acidic groups of at least 2 meq/g, lengths of 0.1 to 10 mm, and diameters of 1 to 50 microns (preferably 10 to 20 microns).
The silver containing adsorbent will advantageously comprise silver containing cation exchange resin or silver containing modified polyacrylonitrile based fibers. The modified polyacrylonitrile based fibers can be obtained in a known manner using alkaline hydrolysis of polyacrylonitrile in the presence of binding agents. They have ion-exchange capacity for basic groups of at least 1 meq/g (milliequivalents per gram), and ion-exchange capacity for acidic groups of at least 2 meq/g, lengths of 1 to 10 mm, and diameters of 1 to 50 microns, preferably 5 to 10 microns.
As water is filtered through the composite material, a layer of practically insoluble silver halogenide compounds is formed on the surface of the granules of iodine containing anion-exchange resin. Release of iodine into water is blocked due to that formation. Complete binding of iodine in the form of practically insoluble silver iodide is guaranteed, because the equivalent content of silver in the material exceeds the equivalent content of iodine.
Excessive silver, as well as excessive iodine is undesirable. Iodine removal is described in many publications; e.g., by passing water through an activated carbon layer. This invention describes using a relatively small amount of silver as compared to prior art methods. Leakage of large amounts of silver into filtered water is thereby avoided. Moreover, any excessive amount of silver (as compared to iodine) interacts with the chloride ions usually present in drinking water, causing formation of silver chloride sediment which becomes attached to the surface of the adsorbents.
The equilibrium content of bactericidal components in solution may be calculated based on the solubilities of the corresponding silver salts. The solubility multipliers (SM) of the silver chloride and silver iodide compounds are, respectively, 1.73xc2x710xe2x88x9210 and 8.1xc2x710xe2x88x9217 (mol/l)2. If, for example, the chloride ion concentration in water is 50 mg/l (1.4xc2x710xe2x88x923 mol/l), then the equilibrium concentration of silver ions in solution is:
[Ag+]=SMAgCl/[Clxe2x88x92]=1.2xc2x710xe2x88x927 mol/l=0.013 mg/l,
and the equilibrium iodide concentration is
[Ixe2x88x92]=SMAgI/[Ag+]=7xc2x710xe2x88x9210 mol/l=8xc2x710xe2x88x928 mg/l.
Thus, equilibrium concentrations of iodide and silver in solution contacting the composite adsorption material are considerably less than the maximum permissible concentrations (World Health Organization xe2x80x9cWater Quality Control Manualxe2x80x9d; xe2x80x9cEuropean Community Drinking Water Directivexe2x80x9d 80/778/EC).
It is believed that the bactericidal effect according to the invention is achieved because the barely soluble bactericidal compounds of silver spread on the developed surface of the material. The granules (the sources of bactericidal substances) are located inside a mesh formed by the amphoteric fibers. The ratio of the size of the granules (1 mm diameter) to the size of the fibers (5-20 micron in diameter) is large. This provides efficient contact between the surface of granules, containing barely soluble silver compounds, and the fibers. Cations of silver and anions of iodide, which are present near the sediment, are adsorbed by the cation and anion exchange groups of the amphoteric fibers, whereupon salt sediment may be formed again. Thus, the sediments of the barely soluble bactericidal compounds, which are initially formed on the surface of the granules (the xe2x80x9csource granulesxe2x80x9d), migrate and gradually spread on the surface of the amphoteric fibers. In accordance with the invention, the surface area of the amphoteric fibers exceeds the exterior surface area of the source granules by at least 100 times, desirably 500 times, and most preferably 800 to 900 times.
In the process of filtration of contaminated water, the microorganisms are adsorbed on the developed surface of the material that contains insoluble bactericidal compounds. The microorganisms contact the bactericides directly, and as the bactericides penetrate through the cellular membrane the microorganisms die. There is no concomitant release of bactericides into the filtered water.
Based on the above description, it is seen that water filtration by the claimed method provides increased time of bacteria contact with the bactericide, thus considerably increasing the life of the sorption column.